1. Field of the Invention
This invention relates generally to the manufacture of dry ice and, more particularly, to a method and apparatus for producing slabs or blocks of dry ice.
2. Background Information
Solid state carbon dioxide (CO2), known as dry ice, is used in many different applications. Dry ice is ideal for preserving food because it sublimates directly from its solid phase to its gaseous phase, leaving no odor, color, taste, or residue and causes no deleterious effects to the food. In cooling and preserving food, dry ice pellets may be placed directly onto the food to rapidly cool it below some specified temperature to prevent spoilage.
Dry ice has traditionally been produced and distributed in blocks with each block weighing about 55 pounds. The blocks are cumbersome, expensive and require extra effort to crush or break apart to make the dry ice easy to use by reducing the block to reasonable size pieces. In recent years, dry ice has been produced in pellet form, which pellets are much easier to use.
A dry ice pelletizer that is made by Tomco Equipment Co. is shown in U.S. Pat. No. 4,780,119, to Brooke, where liquid CO2 is injected into a chamber known as an extrusion chamber and flashed at atmospheric pressure. In this flashing process, part of the liquid CO2 changes phase to a solid known as “snow,” with the remaining part of the liquid CO2 changing phase to gas. The gaseous CO2 can exit the extrusion chamber through gas vents and the remaining snow may be compressed at the end of the extrusion chamber. The proportionate amount of the gaseous CO2 versus the snow depends upon the pressure and temperature of the liquid CO2 that is fed into the extrusion chamber and the surrounding pressure and temperature of the extrusion chamber. The lower the pressure and temperature, the greater the amount of snow produced in the flashing process.
When liquid CO2 is flashed under ideal conditions at atmospheric pressure, approximately 48% of the liquid CO2 is changed to snow, while approximately 52% of the liquid CO2 is changed to gas. Because the percentage of snow formation is directly proportional to the pressure inside the extrusion chamber, when flashing occurs, it is important that the pressure inside the extrusion chamber be kept as close to atmospheric pressure as possible.
Once the snow is formed in the extrusion chamber, a piston is used to compact the snow in one end of the extrusion chamber against a die. In the traditional pelletizer, the snow will collect in the openings of the die and before long block the openings. While some small amount of snow may escape, it is not that significant. Thereafter, when the pistons move back and forth to compress the snow, the snow is compressed at the end against the die to form what is called a puck. As additional dry ice (i.e., snow) is compressed against the puck, the puck will extrude through the openings in the die.
For some applications, the use of pelletized dry ice is not the ideal situation. For example, in some occasions, blocks or slabs of dry ice are much better than pellets of dry ice. However, the 55 pound blocks of dry ice are normally much larger than desired. Sometimes it is necessary to cut the blocks of dry ice into other shapes or sizes, such as shown in U.S. Pat. No. 5,189,939, to Allen. However, when the blocks of dry ice are cut, there is attendant waste in the cutting process.
As an example of an industry that uses smaller blocks or slabs of dry ice, the airline industry uses thousands of pounds of dry ice per day to keep food cool in their serving carts. At the bottom of the serving cart is a tray located a block or slab of dry ice that is approximately 1″×5″×5″. In other words, the 55 pound block would have to be cut into small slabs of dry ice that can be put in the tray in the bottom of the serving cart for the airline industry. This small slab of dry ice will then sublimate directly from the solid to gaseous state leaving no odor and no deleterious effects while keeping the food cool. The airline industry uses large amounts of dry ice per day for this particular purpose.
Slabs or blocks of dry ice could be used for many other purposes other than in the airline industry. Anytime there is a necessity to keep something cool for a period of time in which there is no residue to be dealt with during or after cooling, dry ice becomes an ideal candidate because it sublimates from solid to gaseous state, which gaseous state has no adverse effects.
If smaller blocks or slabs of dry ice can be formed directly from liquid CO2, the losses attendant with cutting of large blocks of dry ice would not occur. The present invention is designed to solve this problem by providing for the extrusion of smaller blocks or slabs of dry ice that can be used in many different applications. None of the devices known by applicant allow for direct extrusion of blocks of dry ice, which blocks could be used by an end user, such as the airline industry.